Generation of a Mn(IV)-Peroxo or Mn(III)-Oxo-Mn(III) Species upon Oxygenation of Mono- and Binuclear Thiolate-Ligated Mn(II) Complexes.

A thiolate-bridged binuclear complex [PPN][(Mn(PS3))] (1, PPN = bis(triphenylphosphine)iminium and PS3H = (2,2',2″-trimercapto-3,3',3″-tris(trimethylsilyl)triphenylphosphine)), prepared from the reaction of MnCl/[PPN]Cl and Li[PS3], converts into a mononuclear complex [PPN][Mn(PS3)(DABCO)] (2) in the presence of excess amounts of DABCO (DABCO = 1,4-diazabicyclo[2.2.2]octane). Variable temperature studies of solution containing 1 and DABCO by UV-vis spectroscopy indicate that 1 and 2 exist in significant amounts in equilibrium and mononuclear 2 is favored at low temperature. Treatment of 1 or 2 with the monomeric O-side-on-bound [PPN][Mn(O)(PS3)] (3) produces the mono-oxo-bridged dimer [PPN][(Mn(PS3))(μ-O)] (4). The electrochemistry of 1 and 2 reveals anodic peak(s) for a MnMn redox couple at shifted potentials against Fc/Fc, indicating that both complexes can be oxidized by dioxygen. The O activation mediated by 1 and 2 is investigated in both solution and the solid state. Microcrystals of 2 rapidly react with air or dry O to generate the Mn(IV)-peroxo 3 in high yield, revealing a solid-to-solid transformation and two-electron reduction of O. Oxygenation of 1 or 2 in solution, however, is affected by diffusion and transient concentration of dioxygen in the two different substrates, leading to generation of 3 and 4 in variable ratios.